The overall objective of the proposed research is to develop enhanced anomalous diffraction methods for analyzing structures of biological macromolecules. We build on our recent accomplishments at New York Structural Biology Center (NYSBC) beamlines at Brookhaven's National Synchrotron Light Source (NSLS) and worldwide experience showing that multiwavelength anomalous diffraction (MAD) and SAD, MAD's single-wavelength counterpart, now predominate for de novo determinations of three- dimensional structures for biological macromolecules. We propose to optimize SAD and MAD phasing procedures by meeting the demands of compelling applications to current problems of biological significance. Biologically exciting problems motivate the development of appropriate tools, and forefront methods accelerate the solution of structures for systems of biological and medical significance. The overall objective is embodied in four specific aims: (1) we propose to enhance SAD phasing procedures for challenging problems such as selenomethionyl proteins at low resolution and only-light-atom native structures. A focus is on improved methods for increasing signal-to-noise ratios for anomalous diffraction by combining data from many crystals. (2) We propose to enhance MAD procedures for accurate experimental phase evaluation. Data collection strategies will be devised to mitigate radiation damage and minimize systematic errors. (3) We propose to develop procedures for low-energy anomalous diffraction experiments. We will develop experimental procedures and design instrumentation for low-energy (3 - 7 keV) experiments aiming to enhance anomalous signal from light atoms such as sulfur and phosphorous. (4) We propose to develop automation and enhanced procedures to facilitate SAD and MAD analyses from many crystals. A first priority is to encode and disseminate our current multi-crystal SAD process in convenient software.